PARIS—Data freshly drawn from the second black box aboard Germanwings Flight 9525 show the plane was repeatedly accelerated as it descended into a mountainside, French aviation investigators said Friday, bolstering prosecutors’ suspicions that co-pilot Andreas Lubitz intentionally crashed the jetliner.

French investigation agency BEA said in a statement that the flight data recorder, which was found and recovered on Thursday, revealed that the person flying the plane at the time of the crash changed the altitude settings...